In manufacturing a semiconductor device, a semiconductor crystal layer is grown, that is, crystal growth is conducted, on a crystal substrate serving as a base. For example, gallium nitride (GaN), which is one of III-V compound semiconductors, is epitaxially-grown on a base crystal substrate including a silicon carbide (SiC) substrate, a silicon (Si) substrate and a sapphire substrate.
It is known that, however, in a case where a lattice constant and/or a thermal expansion coefficient of the base crystal substrate differ from a lattice constant and/or a thermal expansion coefficient of a crystal growth layer that is to be grown, a defect and/or distortion occur to the crystal growth layer, and thus it is difficult that an appropriate semiconductor crystal layer is obtained. Thus, for the purpose of reducing, for example, the defects at the crystal growth layer, a method of growing a semiconductor crystal layer is suggested, in which selective growth is employed. In the selective growth, a substrate formed with masks or with protrusions and recesses is used as the base crystal substrate (refer to JP4743214B and JP4766071B which will be hereinafter referred to as Patent reference 1 and Patent reference 2, respectively).
According to the method described in Patent reference 1, first, growth areas are formed on a substrate surface with the use of a mask material including oxide silicon (SiO2). Next, a GaN-based semiconductor including a facet structure is formed in the growth area formed on the substrate surface. Next, plural GaN-based semiconductors including the facet structures are grown and a dislocation is bent, and the plural GaN-based semiconductors are combined with each other or touch with each other at an upper portion of the mask material or the mask material is covered with the GaN-based semiconductors.
According to the method described in Patent reference 2, a substrate of which a crystal growth surface corresponds to a recess/protrusion surface including protruding portions arranged in a lattice configuration is prepared by, for example, Reactive Ion Etching (RIE). Next, a crystalline layer of AlxGa1-x-yInyN (0≦x≦1, 0≦y≦1) is vapor-grown or vapor-deposited from upper portions of the protruding portions of the substrate. Further, the recess/protrusion surface is covered with the vapor-grown crystalline layer from the upper portions of the protruding portions.
According to the method described in Patent reference 1, however, patterning needs to be applied to the mask material by photolithography and etching, which increases the number of processes or steps, and accordingly increasing costs. In addition, because the mask material is covered with the crystalline layer, the mask material diffuses on the crystalline layer on the mask material. As a result, autodoping, in which the mask material functions as impurity substance in the crystalline layer, may occur.
On the other hand, according to the method described in Patent reference 2, recessed portions of the recess/protrusion surface formed on the substrate are in a surface state in which the crystal can grow thereon. Accordingly, a growth condition of the crystal is extremely restricted in order to prioritize the growth at the protruding portions of the recess/protrusion surface, and thus decline in a crystal quality may be induced. In addition, because the recessed portions are in the surface state that allows the crystal to grow, it is difficult for an appropriate selective growth to occur. Further, abnormal growth may occur due to the crystal growth at the recessed portions.
A need thus exists for a substrate and a manufacturing method thereof, and a semiconductor device, which are not susceptible to the drawback mentioned above.